First, I am a member of the 1750 club, and a level 69 Siren. I have no modded items, and neither does my friend.
I was searching my challenge log in Playthrough 2, and in the money collected, it said I collected -1,771,537,633. Yes, over 1.75 BILLION below zero. My friend had the same thing happen to her, but a slightly higher number. I have since got the total up to 1.5 Billion below zero.
Any ideas on what happened?